mftransform: Fix deadlock when MFT requests processing output twice

mftransform: Fix deadlock when MFT requests processing output twice

This sequence of event/data flow might happen

1) Initially we have one pending output event
  1-1) Then, process the pending output data
2) No pending input event, then we should wait new pending input event
  2-1) Wakeup by new pending event (but it's pending output event)

In above case, MFT will not report new pending input event
if pending output is not processed.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1325>